1. Field of the Invention
The present invention relates to a coating material which is applied to the inner surface of a cathode-ray tube, in particular, to a coating in which filler particles and graphite particles are dispersed in the medium containing water-glass.
2. Description of the Prior Art
Cathode-ray tubes have a funnel glass in which the inner surface is covered with an electrically conductive film. This electrically conductive film is formed by applying to the inner surface of the funnel glass a coating material which contains electrically conductive particles, then drying the coating and heating it in an air.
The above-described coating material is formed, for example, by dispersing and suspending electrically conductive graphite particles and filler particles of metallic oxide or metallic carbide in an aqueous medium which contains a dispersant and water-glass which works for imparting adhesion to the coating material. The metallic oxide or metallic carbide is provided for controlling the electric resistance of the film to an appropriate value, and these compounds include, for example, iron oxide, titanium oxide, silicon carbide and the like.
There is also another type of a coating material which is used for a different type of cathode-ray tubes. This coating material contains graphite but has no metallic oxide or metallic carbide. If the coating material of this type is applied to the funnel glass, a pretty large amount of sparking current flows due to low resistance of the film. Therefore, it is a general manner for cathode-ray tubes to utilize the coating material containing both of the graphite particles and the metallic oxide particles.
In the electrically conductive film made from the above coating material, the graphite works for imparting electrical conductivity to the film and reducing electric resistance of the film. On the other hand, the metallic oxide particles, which are added as a filler, work for raising the electric resistance of the film as well as the water-glass adhesive. Therefore, the blending amounts of the above raw materials are appropriately determined in view of the electric resistance of the film and the adhesion of the coating.
For the manner of applying the coating material to the inner surface of the funnel glass of the cathode-ray tube, spraying or brushing had been conventionally used. However, employment of the flow-coating method is on the increase in recent years, under necessity to improve the manufacturing process of the cathode-ray tubes. In order to employ the flow-coating method, it is necessary to reduce the viscosity of the coating material so that the coating material smoothly flows on the funnel glass of the cathode-ray tube. Specifically, the viscosity range of the coating material which is suitable for the conventional spraying or brushing application is 100 to 200 mPa.multidot.s. In contrast, that for the flow-coating method is about 10 mPa.multidot.s, and this value is quite low. Therefore, even if a coating material in which the viscosity is reduced is obtained by shifting the composition ratio of the conventional coating material, it is difficult to keep the metallic oxide particles suspended in that coating material for a sufficiently long time. This will be also easily understood from Stokes' equation which relates to natural sedimentation of particles. Moreover, in view of storing conditions of the coating material which may easily change, dispersibility of the conventional coating material is more unsatisfactory still.
As described above, it is quite important for the coating material for cathode-ray tubes that the metallic oxide particles are suitably dispersed in the coating material and they are kept suspended in the coating material for a long time so that the life span during which the coating material is usable in the flow-coating system can be extended.
The applicant of the present application has made various researches on improvement of dispersion stability of the particles in the coating material and extension of the life span of the coating material in the case of use in the flow-coating process for the funnel glass. Japanese Patent Publication No. (Kokoku) 63-45428 which was filed by the applicant of the present application and which was published on Sep. 9, 1988 discloses a coating material for cathode-ray tubes. This coating material is prepared by making composite particles which are charged minus as a whole particle, and suspending the composite particles in a water medium containing a binder and a dispersant. The composite particles are manufactured by using a graphite powder, a fine metallic oxide particles and a surface treatment agent which is charged minus. This coating material of the above publication is improved in dispersibillty and dispersion stability. However, this improvement is still insufficient for use in flow-coating system, especially for use at high temperatures.
Moreover, there is another problem in the flow-coating system to efficiently recover and recycle the above-described conventional coating materials. In detail, in the flow-coating process, an excessive amount of the coating material is flowed on the inner surface of the funnel glass, and the over portion of the coating material which flows off the funnel glass is recovered and subjected to stirring treatment for making uniform the recovered coating material. However, if the particles of the coating material are not very durable, the particles in the coating material are broken during the stirring treatment, resulting in cohesion of the dispersed particles.
In addition, it is also necessary for an electrically conductive film obtained from the coating material to firmly adhere to the funnel glass, in order not to deteriorate properties of the cathode-ray tube.
Therefore, a novel coating material in which the dispersion stability is sufficiently improved so that can be utilized in the flow-coating system has been desired.